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Riding his bike to work at Western Washington University, in Bellingham, geophysicist Bernie Housen used to wonder how unhealthy it was to be breathing in all those exhaust fumes.

It's a concern any urban bike rider has at times. But now, thanks to the leaves that grow on the trees along his route, Prof. Housen has proof that his concerns were well founded.

In a remarkable piece of research that could one day lead urban planners to consult the trees on where bike or walking paths should be located, Prof. Housen and his colleague Luigi Jovane measured the levels of magnetism in leaves – and they got some alarming results.

“I ride along one of these high-traffic bus routes and … there was between two and five or six times more magnetic fine particulate matter along the bus route than elsewhere [on less-busy streets],” Prof. Housen said.

The samples taken from urban trees were up to 10 times more magnetic than leaves plucked from rural sites, where there was little traffic.

And there were some real pollution hot spots.

“One of our sample sources was near a little railway switch yard and the magnetism of those leaves was comparable to leaves taken on a street that had 30,000 cars every day going by,” he said.

The magnetism in the leaves is created by tiny particles of iron oxides and other pollutants that drift through the air, coming mostly from diesel exhaust or eroding from the brake pads of vehicles. The particles, so tiny that they would pass directly through a human nasal passage and lodge in the lungs, apparently aren't damaging the plants but could cause health problems for humans.

“I would certainly say that in the very busiest streets there would be cause for concern if you were regularly in these areas,” said Prof. Housen, who now tries to take a less-polluted route when he bikes to campus.

Unfortunately for him, Western Washington University is on a hill top and the bike path that follows the main bus route presents the easiest grade. So, choosing the best route for his lungs means it will be harder on his muscles.

“So it's a trade-off. But I am trying to ride the steeper hills more,” he said.

“One underlying concern is that if you are riding your bike, you are being more physically active; you are breathing deeper and breathing more air in, and so if you are doing that in an area where there is a concentrated elevation of this material it might not be such a good thing,” Prof. Housen said.

Air monitoring has long been standard practice in most North American cities, but Prof. Housen said his tree leaf research opens up a new, perhaps more effective, way of gathering data.

“We found the Bellingham air-monitor station on top of a three-storey building, which is pretty far removed from where pedestrians are, so we are giving a better on-the-ground look at the particle load people actually experience,” he said.

Prof. Housen, who presented his findings at the recent annual meeting of the Geological Society of America, said he's got funding proposals out for more research.

Among other things, he'd like to look at human health impacts, and he'd like to develop a cheaper, easier method for measuring magnetism, so that it would be an affordable tool for urban planners.

“One of the motivations for doing an expanded version of this study would be to gather all the information to better plan bike routes and pedestrian routes and try and separate diesel mass transit from places where people are walking around and breathing a lot of air,” he said.

And that's something for urban bike riders to think about as they commute along routes that, in Vancouver and other cities, are often laid out parallel to busy bus lanes.